[en] This thesis reports about experimental and theoretical investigations of exciting and interesting topics in today's quantum optics and atomic physics. The work considers manifestations of quantum interference effects in laser cooling and manipulation of atoms by laser light, non-linear optics and laser physics. All those effects are related through the phenomenon of coherent population trapping (CPT). First, this work presents analytical and numerical calculations concerned with velocity-selective coherent population trapping (VSCPT). Special attention is paid to possible applications, e.g. the improvement of atomic frequency standards (atomic clocks). Second, this work presents an analysis and numerical simulations of cooperative effects in CPT. Cooperative effects, or superradiance, change dramatically the time dynamics of CPT. The complete set of density matrix equations as well as a new regime of pumping rates has been derived. Quantum interferences have drawn attention in non-linear optics due to the investigation of electromagnetically induced transparency (EIT). The final chapter reports about an EIT experiment in order to study continuous light wave propagation through dense sodium vapour. Additionally, a theory based on simple models of EIT has been developed. (author)